Melting line of calcium characterized by in situ LH-DAC XRD and first-principles calculations

Simone Anzellini,Dario Alfé,Daniel Errandonea,Monica Pozzo

doi:10.1038/s41598-021-94349-4

Abstract

In this work, the melting line of calcium has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using first-principles calculations. In the investigated pressure and temperature range (pressure between 10 and 40 GPa and temperature between 300 and 3000 K) it was possible to observe the face-centred phase of calcium and to confirm (and characterize for the first time at these conditions) the presence of the body-centred cubic and the simple cubic phase of calcium. The melting points obtained with the two techniques are in excellent agreement. Furthermore, the present results agree with the only existing melting line of calcium obtained in laser-heated diamond anvil cells, using the speckle method as melting detection technique. They also confirm a flat slope of the melting line in the pressure range between 10 and 30 GPa. The flat melting curve is associated with the presence of the solid high-temperature body-centered cubic phase of calcium and to a small volume change between this phase and the liquid at melting. Reasons for the stabilization of the body-centered face at high-temperature conditions will be discussed.

Highlights

The phase diagrams and melting behaviour of the alkaline-earth metals at extreme conditions of pressure (P) and temperature (T) are of fundamental interest in condensed matter physics and material science
Among the high P–T studies on Ca, only a recent study by Anzellini et al.[6] has used X-ray diffraction (XRD) to unambiguously determine the observed crystal structures. In this particular study, performed using a resistively-heated diamond anvil cell (DAC) coupled with synchrotron XRD, the high P–T phase diagram of Ca was characterized between ambient conditions and 53 GPa—800 K
Five experimental runs have been performed at Diamond Light Source (DLS), using the experimental conditions reported in the “Methods” section

Summary

Introduction

The phase diagrams and melting behaviour of the alkaline-earth metals (group IIa in the periodic table) at extreme conditions of pressure (P) and temperature (T) are of fundamental interest in condensed matter physics and material science. Among the high P–T studies on Ca, only a recent study by Anzellini et al.[6] has used X-ray diffraction (XRD) to unambiguously determine the observed crystal structures. In this particular study, performed using a resistively-heated diamond anvil cell (DAC) coupled with synchrotron XRD, the high P–T phase diagram of Ca was characterized between ambient conditions and 53 GPa—800 K. High Pressure (MALTA) Consolider Team, Universidad de Valencia, Edificio de Investigación, C/Dr Moliner 50, Scientific Reports | (2021) 11:15025 In this experiment, the pressures were determined ex situ using the ruby fluorescence method[18] and the temperature was determined using spectral radiometry[19]. The observed behaviour suggested that below 32 GPa Ca melts from the Ca-II (bcc) phase and that there must be a Ca-II–Ca-III-liquid (bcc-sc-liquid) triple point located around 32 GPa and 1550 K

Methods

Results

Conclusion